1. Field of the Invention
The present invention relates to an automatic exposure control apparatus for a camera. More particularly, the present invention relates to an automatic exposure control apparatus suitable for a camera having an automatic focus adjusting device.
2. Description of the Prior Art
Various automatic focusing apparatus have been proposed and used in the art. However, when such automatic focusing apparatus is incorporated in a camera, problems in respect of focus detection accuracy (resolving power) arise. Because of limitations in detection accuracy, it is difficult to fully enjoy the merit of built-in automatic focus adjusting apparatus. As a solution to the problem, it has been proposed to compensate the insufficient resolving power by making use of the depth of focus which can be changed by changing the diameter of the diaphragm aperture. In other words, when a picture is taken using an automatic focusing apparatus, it has been proposed to select a diaphragm value which may render the depth of focus larger than a determined value.
Automatic exposure control apparatus available at present may be classified generally into three types, namely shutter priority automatic exposure control system, diaphragm priority automatic exposure control system and programmed automatic exposure control system.
In the case of automatic exposure control of shutter priority AE mode, the shutter time is fixed constant. A proper exposure is obtained by changing the diaphragm value according to the luminance of the object. Therefore, for a dark object, the diaphragm may be set to a very small value (a very large diaphragm aperture). Similarly, in the case of programmed AE mode, a smaller diaphragm value is used for a darker object.
If an automatic focusing apparatus is incorporated into such an automatic exposure control type of camera, then difficulties arise in taking a picture of a dark object. When the object is dark, the depth of focus resulting from the diaphragm value then automatically set becomes small. To adjust the focus within the given small depth of focus, an extremely high precision is required for the focus detection part and also the lens positioning mechanism in the camera. This involves great technical difficulties and also intolerably high cost. Furthermore, there is the possibility that the focus is no longer adjustable to a point within the depth of focus, and the picture may be out of focus although the exposure is proper.